Field of the Invention
This invention relates generally to a thin film, flexible antenna configured on a dielectric substrate and, more particularly, to a thin film, flexible, leaky-wave co-planar waveguide (CPW) antenna that may include transparent conductors so as to allow the antenna to be adhered to a visible part of vehicle glass.
Discussion of the Related Art
Modern vehicles employ various and many types of antennas to receive and transmit signals for different communications systems, such as terrestrial radio (AM/FM), cellular telephone, satellite radio, dedicated short range communications (DSRC), GPS, etc. The antennas used for these systems are often mounted to a roof of the vehicle so as to provide maximum reception capability. Further, many of these antennas are often integrated into a common structure and housing mounted to the roof of the vehicle, such as a “shark-fin” roof mounted antenna module. As the number of antennas on a vehicle increase, the size of the structures required to house all of the antennas in an efficient manner and providing maximum reception capability also increases, which interferes with the design and styling of the vehicle. Because of this, automotive engineers and designers are looking for other suitable areas on the vehicle to place antennas that may not interfere with vehicle design and structure.
One of those areas is the vehicle glass, such as the vehicle windshield, which has benefits because glass typically makes a good dielectric substrate for an antenna. For example, it is known in the art to print AM and FM antennas on the glass of a vehicle where the printed antennas are fabricated within the glass as a single piece. However, these known systems are generally limited in that they can only be placed in a vehicle windshield or other glass surface in areas where viewing through the glass is not necessary.
Cellular systems are currently expanding into 4G long term evolution (LTE) that requires multiple antennas to provide multiple-input multiple-output (MIMO) operation, which provides greater data throughput and bandwidth than previous cellular communications technologies, such as 2G and 3G. LTE 4G cellular technology employs MIMO antennas at the transmitter and the receiver that provide an increase in the number of signal paths between the transmitter and the receiver, including multipath reflections off of various objects between the transmitter and the receiver, which allows for the greater data throughput. As long as the receiver can decouple the data being received on each path at the MIMO antennas where the signals are uncorrelated, then those paths can be used by the receiver to decipher data transmitted at the same frequency and at the same time. Thus, more data can be compressed into the same frequency providing higher bandwidth.
Automobile manufacturers are looking to provide 4G cellular technology in vehicles, which presents a number of design challenges especially if the MIMO antennas are incorporated as part of a common antenna structure mounted to the roof of the vehicle. For example, by housing the MIMO antennas, which include at least two antennas, in the traditional telematics antenna module mounted to the roof of the vehicle, the entire antenna volume of the module would need to increase because of the extra real estate required for the MIMO antennas, which require a low correlation of the received signals at the antennas. In other words, because the signals received by the MIMO antennas need to be significantly uncorrelated, the distance between the antennas needs to be some minimum distance depending on the frequency band being employed. This de-correlation between the antenna ports is often times difficult to achieve in various designs if the antenna elements are located at the same general location because the signals received at the port would be very similar. This problem can be overcome by moving the antennas farther apart, such as placing the antennas on the vehicle glass.
For those antennas that are adhered to the vehicle windshield or rear window, the curvature of the window causes the radiation pattern of the antenna to be directed more upward rather than parallel to the ground. Because the radiation pattern is directed upward in this manner, the transmission and reception direction of the antenna is often not specifically directed towards the desired receiver or transmitter, and thus signal loss can occur.